The present invention relates generally to marine seismic exploration, and more particularly to a marine seismic measurement system that allows for attenuation of free surface multiples in two-component marine bottom sensor data.
Marine seismic wave measurement systems are used to take seismic profiles of underwater geological configurations. One procedure of marine seismic measurement involves the use of a marine bottom cable. Surveys using marine bottom cables are often employed in areas that are populated with numerous obstacles, such as drilling and production platforms. In a procedure using marine bottom cable, bottom cables are deployed along the marine bottom. Often, multiple cables are deployed in parallel. Each bottom cable has a plurality of sensor pairs placed at regular intervals along the cable. Each sensor pair contains a pressure sensor, such as a hydrophone, and a particle velocity sensor, such as a geophone. A gimbal mechanism within each geophone ensures that the sensing elements of the geophones are vertically oriented.
Acoustic energy is generated in the vicinity of the marine bottom cables using an acoustic energy source such as an air gun array or a marine vibrator array. An air gun discharges air under very high pressure into the water. Marine vibrators typically include a pneumatic or hydraulic actuator that causes an acoustic piston to vibrate at a range of selected frequencies. The vibrations of the acoustic vibrator produce pressure differentials in the water that generate acoustical energy pulses. Source acoustical waves travel downward through the water and into the earth as seismic waves. The source waves strike interfaces between formations in the earth. A portion of the source wave is reflected upwards from the interface towards the marine bottom. The sensor array on the marine bottom receives the reflected waves and converts the waves into signals that are recorded as sensor data. The sensor data is processed to provide information about the structure of the formations beneath the marine bottom.
The sensor array receives not only the reflected waves of interest, but also the source waves and reverberated waves. Reverberated waves are waves that have been reflected from the water-air interface back towards the marine bottom. Such reverberated waves are referred to as free surface multiples or surface multiples. The free surface multiples may be significant in amplitude and may be difficult to differentiate from the desired reflected waves.
The use of dual sensor measurements, namely pressure and vertical particle velocity, allow for the attenuation of free surface multiples. U.S. Pat. Nos. 5,163,028; 5,365,492; 5,524,100, and 5,621,700 describe methods of attenuating free surface multiples, and each of these patents are incorporated by reference as if fully set forth herein. The methods of attenuating free surface multiples detailed in the above referenced patents do not adequately take into consideration the angle dependence of the upgoing and downgoing wavefields. Also, the methods of attenuating multiples detailed in the above referenced patents do not adequately take into consideration the angle dependency of the response of each sensor of a sensor pair. The use of methods of attenuating multiples that do not consider both the angle dependency of the upgoing and downgoing wavefields and the angle dependency of the response of each sensor of a sensor pair may lead to inaccurate determinations of the formations present beneath the marine bottom.
Existing methods of deconvolution for multiple attenuation of dual sensor data carry out the calibration and deconvolution filter in the distance-time domain, (x,t). The basic equations for deconvolution of upgoing and downgoing waves are valid only in the angle (plane wave) domain. Seismograms recorded from a single shot will have energy propagating at all possible angles, so processing data in the distance-time domain can only have limited success.
The problems outlined above are in large part to be solved by a system and method of marine seismic exploration that takes into account angle dependencies during the processing of two-component sensor data. Consideration of the angle dependencies of upgoing wavefields, downgoing wavefields, and the sensor enhance the attenuation of free surface multiples that are present in two-component sensor data. The ability to provide enhanced attenuation of free surface multiples allow for more accurate determination of the formations present beneath a marine bottom.
In one example embodiment, a method is used to decompose the recorded dual sensor data into upgoing and downgoing wavefields in the plane wave domain. The method finds an angle dependent calibration factor that allows the calibration of the recorded pressure data with respect to the recorded vertical particle velocity data. The angle dependent calibration factor takes into consideration the angle dependencies of the hydrophone and the geophone, as well as noise associated with the recording geometry. Attenuation of multiples is accomplished by application of an incident angle dependent deconvolution of the downgoing wavefield from the upgoing wavefield calculated from the calibrated pressure and vertical particle velocity data.
In a further embodiment, a method of attenuation of multiple reflections in seismic data is provided. The seismic data comprises pressure data and particle velocity data. The method comprises deconvolving the seismic data, and applying a moving average operator to the seismic data.
In a further embodiment, a system for attenuation of multiple reflections in seismic data is provided. The seismic data comprises pressure data and particle velocity data. The system comprises means for deconvolving the seismic data, and means for applying a moving average operator to the seismic data.
In a further embodiment, seismic data is provided. The seismic data is processed by a method comprising deconvolving the seismic data, and applying a moving average operator to the seismic data.